A control system of this kind is known from German patent 37 11 049 in which each individual pump has associated with it a measuring and control unit comprising a pressure limiting valve, a position detector and a line incorporating a throttle. This line connects the inlet of the pressure limiting valve with a control pressure line connected to the working pressure line of the respective pump and leads to a spring chamber of the pilot valve, which is in the form of a 3/2-way valve. A spring in the spring chamber produces the counter-pressure that is opposed by the working pressure prevailing in the working pressure line and in the consumer line as a first control pressure. The position detector comprises an inclined surface formed on the piston rod of the adjusting piston of the adjusting cylinder and a feeler pin that is held against the oblique face by a spring, through which it acts on the spool of the pressure limiting valve. If the associated pump is adjusted to zero displacement the pressure limiting valve is closed, producing a correspondingly large second control pressure. Increasing the tilting-out of the pump leads to a magnified response corresponding to the movement of the oblique face, reducing the second control pressure.
As the displacement of the pump decreases the thus-controlled second control pressure in the spring chamber of the pilot valve increases the pressure difference between the first and second control pressures that acts against the counter-pressure as the intended value of the control variable. This results in a p-Q control characteristic that rises as the pump displacement decreases and deviates from the intended value that corresponds to the constant counter-pressure by the so-called residual intended pressure deviation. Such pressure-regulated pumps exhibit so-called proportional behaviour and consequently, inter alia, the advantage that with the same setting of the counter-pressure, despite different starting positions, they can be regulated to the same displacement setting, i.e. in the case for example of axial piston machines to the same tilt angle, provided their adjusting means have the same adjusting characteristic. However, despite their displacement settings being the same, the actual output of the individual pumps may be different because of disturbing factors such as, for example, different pump rotating speeds, different constructional tolerances, different frictional and internal forces, and also differing adjusting characteristics of the adjusting means, etc. A further disadvantage of the known control system is the high constructional outlay on the measuring and control units needed to generate the second control pressure.